1. Field of the Invention
This invention relates to a novel method of producing various types of modified rosin resin esters of polyhydric alcohols. These modifications include the addition of maleic anhydride or other unsaturated dibasic acids to rosin. In particular, this invention relates to reacting rosin and the unsaturated dibasic acid in the presence of very low quantities of phosphinic acid, as a catalyst. The modified rosin is subsequently reacted with one or more various polyhydric alcohols to form modified rosin resins having improved properties such as color, softening point and viscosity in a specified solution.
2. Description of the Prior Art
Rosin is mainly a mixture of C.sub.20, fused-ring, monocarboxylic acids, typified by levopimaric and abietic acids, both of which are susceptible to numerous chemical transformations. The rosins to which this invention relates include gum rosin, wood rosin and tall oil rosin.
The natural separation and gradual conversion of some of the hydrophilic components of sap and related plant fluids from the cambium layer of a tree into increasingly hydrophobic solids are the generic process of forming diverse gums, resins and waxes. The oleoresin intermediate in this process is typified in pine gum, which flows from hacks on the trunks of southern yellow pine primarily in Portugal, Brazil and China, as well as and in other countries. Pine gum contains about 80% (gum) rosin and about 20% turpentine.
Resinification from oleoresin can result from either natural evaporation of oil from an extrudate or slow collection in ducts in sapwood and heartwood. Pinus stumps are valuable enough to be harvested, chipped, and extracted with hexane or higher-boiling paraffins to yield wood rosin, wood turpentine, and other terpene-related compounds by fractional distillation. In the kraft, i.e., sulfate, pulping process for making paper, pinewood is digested with alkali producing crude tall oil and crude sulfate turpentine as by-products. Fractionation of the crude tall oil yields tall oil rosin and fatty acids.
Prior to this invention dibasic modified rosin esters of polyhydric alcohols were produced by adducting as much as 30% (based on the weight of the rosin) fumaric acid or maleic anhydride to the rosin by thermally driving the reaction to completion. Reaction times as long as four (4) hours could be expected when performing this addition at 180.degree. C. to 210.degree. C. Fumaric acid additions take a longer time to complete than maleinized adducts; however, maleic anhydride adducts yield a darker colored product. High levels of unsaturated dibasic acids are used to give the resin the basis for the high molecular weights needed to meet certain performance criteria such as hardness, or softening point, viscosity, and rub resistence, as well as other specific end use demands.
Also incorporated into some resins is the addition to the rosin of a phenolic-formaldehyde condensate. Prior to this invention it has been difficult to form an adduct of most common grades of rosin by reacting the rosin with a smaller level of a phenolic-formaldehyde, followed by a higher level of fumaric acid. The nature of this problem is not definitely known, but it is theorized that the rosin-phenolic condensation product somehow blocks the fumaric addition reactions.
This problem is not evident when the phenolic is followed by a maleic anhydride addition, but again the maleic adduct gives darker colors which are unacceptable for use in the application for which the resin is intended. The phenolicformaldehyde condensate addition product contributes better drying speed to the resin.
The above mentioned modified rosins upon completion are then esterified with one or more polyhydric alcohols such as pentaerythritol, glycerine, sorbitol, or trimethylolpropane. The esterification reaction is carried out at temperatures of 215.degree. C. to 280.degree. C. depending upon the polyalcohol used. The esterification reaction may be carried out to an acid number of 220 to 15, depending upon the level of modification of the rosin and the end use for which the resin is intended. This esterification step can be carried out at atmospheric or reduced pressure typically with the use of a calcium hydroxide catalyst.
The beneficial product characteristics provided by rosin esterification for various applications have led to the development of many esterification procedures, particularly treatments with polyhydric alcohols. U.S. Pat. Nos. 2,369,125, 2,590,910 and 2,572,086 teach rosin esterification with glycerol and pentaerythritol, among other polyhydric alcohols, usually preceded by a rosin disproportionation step.
It is generally known in the art that a significant disadvantage of pentaerythritol esterification of tall oil rosin is the deterioration of rosin color in the ester product. For a tall oil rosin with a starting color of 8 on the Gardner scale, a pentaerythritol ester would have a color of 13-18 while a glycerol ester would have a color of 8-9. Also, extremely long reaction times are required to make the tall oil rosin-pentaerythritol esters (up to 30-48 hours) as compared to making tall oil rosin-glycerol esters under identical conditions (10-12 hours).
U.S. Pat. Nos. 3,780,012 and 3,780,013 acknowledge that tall oil rosin darkens significantly upon pentaerythritol esterification and propose alternative solutions. U.S. Pat. No. 3,780,012 teaches pretreating the rosin with catalytic amounts of paraformaldehyde followed by distillation prior to the esterification reaction. U.S. Pat. No. 3,780,013 teaches the incremental addition of a phenol sulfide compound during the esterification. The color of the product of these procedures was claimed to be an M on the U.S.D.A. scale. Also, the patents' examples employed a 20% equivalent excess of pentaerythritol.
U.S. Pat. No. 2,729,660 also acknowledges the darkening effect which common esterification catalysts such as strong acids cause on the product during esterification. The patent teaches the use of 0.5 to 5% of either the aliphatic or aromatic esters of phosphorous acid as a catalyst for the esterification of higher fatty acids or rosin acids, or mixtures thereof. In addition to avoiding appreciable color formation during the esterification, a reduction in reaction time is noted. A distinct disadvantage of the process is the dissociation, during esterification, of the alcohol used to make the phosphite ester catalyst resulting in a disagreeable odor.
Also, U.S. Pat. No. 4,172,070 teaches employing arylsulfonic acid in place of the traditional basic esterification catalysts, such as calcium oxide, to reduce the time for tall oil rosin-pentaerythritol esterification to obtain a rosin ester of improved oxygen stability, color and softening point. This work is confounded, however, by the unusually large amount of pentaerythritol used (35% equivalent excess) which by itself would markedly increase the rate of acid number drop. Products with Ring and Ball softening points of 77.degree. C. to 86.5.degree. C. were obtained. Normal commercial pentaerythritol esters of rosins soften between 95.degree. C. and 105.degree. C.
The primary object of this invention is to provide a novel method of preparing the above mentioned phenolic dibasic modified rosin resins that yields products with lighter colors without adversely affecting viscosities and softening points and facilitates the addition of a phenolic-formaldehyde condensate prior to a fumaric acid addition to the rosin.